My research goal is to become an independent investigator that heads a research group aimed at understanding host immunity against vacuolar pathogens. To this end, I am using the respiratory pathogen Legionella pneumophila as a model organism to understand immune detection of pathogens and subsequent control of infection. Upon host cell infection, L. pneumophila is able to prevent its transport to lysosomes, and remodels its vacuole into an endoplasmic reticulum-derived compartment that supports bacterial replication. To accomplish this, L. pneumophila utilizes a type IV secretion system (T4SS) to translocate bacterial effectors into the host cell. We have found that the host is able to discriminate between virulent and avirulent bacteria. We have identified host MAPK pathways activated only in response to virulent bacteria expressing a functional T4SS that result in an increased proinflammatory cytokine response. This T4SS-dependent host response is distinct from TLR, Nod1, Nod2, and inflammasome signaling. However, little is known about the identity of the bacterial factors that trigger this response and how this pathway contributes to immune gene expression and subsequent control of bacterial infection in vivo. A multi-disciplinary approach will be taken to answer these questions. In Aim 1, I will identify and characterize the bacterial components required for T4SS-dependent activation of host MAP kinases. In Aim 2, I will identify and characterize the contributions of TLR-dependent and T4SS-dependent signaling to immune gene expression. Finally, in Aim 3, I will dissect the contributions of various cell types to the innate immune detection of L. pneumophila and subsequent control of infection. During the mentored phase, completion of Aim 1 will provide a series of relatively straightforward experiments as well as screen-based approaches that will yield bacterial candidates that can be pursued during the independent phase. Additionally, bacterial strains described in Aim 3 will be generated and mouse breeding for Aim 3 will be started. During the independent phase, Aims 2 and 3 involving molecular immunology and in vivo infection models will be pursued. These studies will provide a solid foundation for further investigations into understanding host responses to vacuolar pathogens that will be pursued during the independent phase of my career. 7. Project narrative Understanding host responses to the respiratory pathogen L. pneumophila will advance our understanding of bacterial virulence and how the innate immune system distinguishes between virulent and avirulent bacteria and initiates antimicrobial immunity. This will ultimately aid in the design of effective antimicrobial therapies and vaccines.